Tracheal Bifurcation (T4/T5)

The tracheal bifurcation is the point at which the trachea divides into the right and left main bronchi. In an adult, this division is commonly located near the transverse thoracic plane passing through the sternal angle anteriorly and the T4-T5 intervertebral disc posteriorly.

The internal ridge at the bifurcation is called the carina. Although the bifurcation cannot be palpated directly, the readily palpable sternal angle provides an important surface landmark for estimating its level.

The exact position of the bifurcation is not fixed. It moves with respiration and can vary with posture, age, body habitus, and changes in thoracic anatomy.

Location

The tracheal bifurcation lies within the thorax near the junction between the superior and inferior mediastina.

It occurs inferior to the cervical and upper thoracic portions of the trachea and marks the beginning of the bronchial tree supplying the two lungs.

Surface Projection

On the anterior chest wall, the tracheal bifurcation is projected approximately to the level of the sternal angle.

Posteriorly, this corresponds approximately to the intervertebral disc between the fourth and fifth thoracic vertebrae.

Sternal Angle

The sternal angle, or angle of Louis, is formed at the manubriosternal joint.

It is a palpable transverse ridge and provides the principal external reference for the approximate level of the tracheal bifurcation.

T4-T5 Level

The transverse plane passing from the sternal angle posteriorly reaches approximately the T4-T5 intervertebral disc.

For this reason, the tracheal bifurcation is commonly described as lying at the T4-T5 level in the adult.

Transverse Thoracic Plane

The horizontal plane through the sternal angle and T4-T5 intervertebral disc is called the transverse thoracic plane.

It is an important anatomical reference because several major thoracic structures and transitions are associated with this level.

Structures Related to the Transverse Thoracic Plane

Structure or Event	Relationship
Tracheal bifurcation	Usually lies near this plane
Carina	Located at the tracheal division
Second costal cartilage	Articulates with sternum at the sternal angle
Aortic arch	Beginning and end classically related to this level
Mediastinum	Plane separates superior from inferior mediastinum
Vertebral level	Approximately T4-T5 intervertebral disc

Carina

The carina is a keel-shaped cartilaginous ridge located internally at the point where the trachea divides.

It separates the openings of the right and left main bronchi and is an important landmark during bronchoscopy.

Right Main Bronchus

The right main bronchus extends from the tracheal bifurcation toward the right lung.

It is generally wider, shorter, and more vertically oriented than the left main bronchus.

Left Main Bronchus

The left main bronchus extends from the bifurcation toward the left lung.

It is generally longer, narrower, and more oblique than the right main bronchus because of the arrangement of the heart and great vessels.

Main Bronchi Comparison

Feature	Right Main Bronchus	Left Main Bronchus
Length	Shorter	Longer
Width	Wider	Narrower
Orientation	More vertical	More oblique
Destination	Right lung	Left lung

Relationship to the Trachea

The trachea descends from the neck into the superior mediastinum before dividing into the main bronchi.

Its cartilaginous framework maintains airway patency while the posterior membranous wall allows some flexibility.

Relationship to the Esophagus

The esophagus lies posterior to the trachea and continues posterior to the region of the tracheal bifurcation.

The close relationship between the airway and esophagus is important in mediastinal anatomy and cross-sectional imaging.

Relationship to the Aortic Arch

The aortic arch lies to the left of the trachea and passes over the region of the left main bronchus.

The beginning and end of the arch are classically related to the transverse thoracic plane near the level of the tracheal bifurcation.

Relationship to the Pulmonary Trunk

The pulmonary trunk arises from the right ventricle and divides into right and left pulmonary arteries within the central thorax.

Its bifurcation lies inferior to the tracheal bifurcation and is closely related to the main bronchi and other structures at the lung roots.

Relationship to the Pulmonary Arteries

The pulmonary arteries pass from the pulmonary trunk toward the respective lung roots.

Their relationship to the main bronchi differs on the two sides and is an important feature of hilar anatomy.

Relationship to the Mediastinum

The tracheal bifurcation lies near the boundary between the superior mediastinum and the middle mediastinal region below.

The transverse thoracic plane is used as the conventional division between the superior and inferior mediastina.

Superior Mediastinum

The thoracic trachea descends through the superior mediastinum before reaching its bifurcation.

The superior mediastinum also contains major vessels, nerves, lymphatic structures, the esophagus, and thymic remnants.

Respiratory Movement

The position of the tracheal bifurcation changes during respiration.

With deep inspiration, the bifurcation may descend as the lungs expand and the thoracic dimensions increase.

Postural Variation

Body position can influence the apparent level of the carina and tracheal bifurcation.

For this reason, the T4-T5 and sternal-angle relationships are best regarded as standard surface approximations.

Age-Related Variation

The position of the tracheal bifurcation differs with age and thoracic proportions.

In children and infants, airway dimensions and relationships differ substantially from those of adults.

Surface Landmark Technique

1. Locate the jugular notch at the superior border of the manubrium.
2. Move inferiorly along the sternum.
3. Identify the palpable ridge of the sternal angle.
4. Recognize this level as the approximate anterior projection of the T4-T5 transverse thoracic plane.
5. Use this plane as an external estimate of the tracheal bifurcation.

Relationship to the Second Rib

The second costal cartilage articulates with the sternum at the sternal angle.

This allows the same landmark used to estimate the tracheal bifurcation to serve as the standard starting point for counting ribs and intercostal spaces.

Bronchial Tree

After the tracheal bifurcation, each main bronchus enters its respective lung and divides into lobar bronchi.

The lobar bronchi subsequently divide into segmental bronchi supplying the bronchopulmonary segments.

Airway Branching

Level	Airway Structure
Central airway	Trachea
First division	Right and left main bronchi
Within lungs	Lobar bronchi
Further division	Segmental bronchi

Clinical Significance

Bronchoscopy

During bronchoscopy, the carina is an important internal landmark.

Visualization of the carina helps orient the examiner to the openings of the right and left main bronchi.

Endotracheal Tube Position

The relationship between the tip of an endotracheal tube and the carina is important when assessing airway placement.

Chest imaging is commonly used to confirm that the tube tip is appropriately positioned above the carina.

Right Main Bronchus Intubation

If an endotracheal tube advances too far beyond the tracheal bifurcation, it may preferentially enter a main bronchus, commonly the right because of its more vertical course.

This can result in inadequate ventilation of the opposite lung.

Aspiration

The more vertical orientation of the right main bronchus contributes to the tendency for aspirated material to enter the right bronchial tree more frequently in many clinical circumstances.

The eventual location also depends on body position and airway anatomy.

Carinal Widening or Distortion

Changes in the appearance or angle of the carina on imaging may occur when neighboring mediastinal structures are enlarged or displaced.

Such findings require interpretation together with the full clinical and imaging context.

Lymph Nodes

Tracheobronchial lymph nodes are concentrated around the tracheal bifurcation and main bronchi.

These nodes are important pathways for lymphatic drainage from the lungs and can become enlarged in infectious, inflammatory, or neoplastic disease.

Airway Obstruction

Lesions near the tracheal bifurcation can potentially affect airflow to one or both main bronchi.

The central location of the carina makes this region particularly important in airway assessment.

Imaging

The carina and main bronchi can be identified on chest radiographs and are demonstrated in greater anatomical detail with CT imaging.

Imaging allows the level, airway caliber, adjacent structures, and position of tubes or devices to be assessed directly.

Chest Radiography

On a frontal chest radiograph, the tracheal air column can often be followed inferiorly to the carina.

The carina serves as an important reference when assessing central airway anatomy and endotracheal tube placement.

Computed Tomography

CT provides cross-sectional visualization of the tracheal bifurcation, main bronchi, mediastinal vessels, lymph nodes, esophagus, and surrounding tissues.

It demonstrates why a single surface landmark represents only an approximate external projection of this three-dimensional region.

Tracheal Bifurcation Versus Sternal Angle

Feature	Tracheal Bifurcation	Sternal Angle
Type	Internal airway division	Palpable skeletal landmark
Location	Central thorax	Anterior thoracic wall
Approximate level	T4-T5 region	T4-T5 transverse plane
Key structure	Carina	Manubriosternal joint
Surface use	Estimated indirectly	Provides external reference

Key Surface Anatomy Features

Feature	Key Point
Structure	Division of trachea into main bronchi
Internal ridge	Carina
Anterior surface landmark	Sternal angle
Approximate vertebral level	T4-T5 intervertebral disc
Anatomical plane	Transverse thoracic plane
Right branch	Wider, shorter and more vertical main bronchus
Left branch	Longer, narrower and more oblique main bronchus
Movement	Position varies with respiration
Mediastinal significance	Near plane separating superior and inferior mediastina
Clinical importance	Bronchoscopy, airway tubes, aspiration and imaging

Anatomical Importance

The tracheal bifurcation provides an important link between thoracic surface anatomy and central airway anatomy. Although the bifurcation itself cannot be palpated, its approximate level can be estimated using the sternal angle, which corresponds to the transverse thoracic plane near the T4-T5 intervertebral disc.

At the bifurcation, the carina separates the openings of the right and left main bronchi. The differing orientation of these bronchi is clinically important, particularly in aspiration, bronchoscopy, and endotracheal intubation.

The T4-T5 relationship should be treated as an approximate anatomical landmark because the bifurcation moves with respiration and varies with posture, age, and individual anatomy. Even so, the sternal angle remains one of the most useful external reference points for understanding the surface projection of the tracheal bifurcation and neighboring mediastinal structures.